Microstructural characterization of the ζ- and γ-hydride phases in Zircaloy-2 by Electron Diffraction and Energy-Loss Spectroscopy Techniques.

Abstract

In this study, the formation of the ζ- and γ-hydride phases in the form of independent bulk precipitates and interfacial ribbons between δ-precipitates and α-Zr matrix was investigated.

Energy-Loss Spectroscopy (EELS) and Nano-Beam Electron Diffraction (NBED) techniques were utilized to characterize nano-hydrides formed in water-quench and furnace-cool conditions. For δ-nano-hydrides, EELS measured two ~10 nm-wide ribbons with plasmon energy (PE) values of 17.4±0.3 eV and ~18.3±0.3 eV (characteristic of the ζ- and γ-phases, respectively) surrounding the δ-core. Complementary NBED characterization of multiple zone axes of the interface, however, did not suggest the existence of the ζ- or γ-phases in the interface.

Next, it was attempted to clarify why the characteristic PE values of the ζ- and γ-phases were observed in the interface. For this purpose, energy-loss spectra of the α-Zr and δ-hydride phases were simulated in interfacial areas between the two phases. Simulations predicted a gradual shift of PE between 16.9-19.2 eV over a ~5 nm distance due to the interface effect. In addition, a delocalization length of ~16 nm was measured for the 16-20 eV energy-loss window. Results showed that the observed interfacial ribbons stemmed from a combination of the interface effect and the delocalized nature of the plasmon vibration, and not from the formation of interfacial ζ- and γ-ribbons.

In the next step, the formation of the ζ- and γ-phases as bulk precipitates was investigated. Synchrotron X-ray diffraction examination of water-quenched hydrides revealed a diffraction peak at the d-spacing value ~2.70 Å, which can stem from either the (0004)ζ or {111}γ planes. In a quest for the bulk ζ-precipitates, nano-hydrides were characterized in [0001], <112̅6>, <101̅4>, and <112̅0> orientations where NBED patterns identical to those reported for the ζ-phase in previous works were collected. Analysis of electron diffraction patterns revealed that reflections that are conventionally attributed to the ζ-phase, in fact, originate from either the δ-phase covered with a thin surface phase (probably Zr-oxide) or dynamical scattering events between the α-Zr and δ-hydride in overlapped areas. Finally, EELS and NBED detected only the γ- and δ-phases in the microstructure, but not the ζ-phase.